Guidelines for zirconium tolerance in Li-ion battery cathode recycling from manufacturing scraps

As electric vehicles (EVs) flourish, the volume of recyclable battery resources is also increasing, which can be broadly categorized into spent lithium-ion batteries (LIBs) and manufacturing scraps. Zr as a common doping/coating element for cathode active materials could exist as an impurity in resynthesized LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) from cathode scraps. Considering that the Zr content in leachate from cathode scraps is around 0.25 mol% (vs. Ni + Co + Mn), we synthesize Zr-incorporated samples with Zr contents of 0, 0.1, 0.5, and 1 mol%, which are designated as NCM622, NCMZr 0.1, NCMZr 0.5, and NCMZr 1.0, respectively. As the Zr content increases, the degree of cation mixing increases, leading to a reduction in the initial charge-discharge capacity in a cut-off voltage of 4.3 V. The rate capability also declines, as confirmed by a decrease in Li⁺ diffusion coefficient. On the other hand, the cyclability of Zr-doped NCM significantly improves, owing to strong Zr-O bonding energy and the pillar effect. A moderate Zr content (∼0.5 mol%) shows the best structural stability and cyclability with better reversibility and lower overvoltage. In contrast, the excessive Zr⁴⁺ content (>1 mol%) does not only offer additional benefits but also hinders performance due to the electrochemical inactivity of Zr. This study provides a guideline for the acceptable tolerance level of Zr impurity in the resynthesis of cathode active materials from manufacturing scraps, contributing to the economic feasibility of LIB recycling.